This invention relates to a process for bleaching, i.e. improving the color properites of, kaolin clay. In a particular aspect this invention relates to an improved process for bleaching kaolin wherein sodium dithionite is used as the bleaching agent.
Large amounts of kaolin clay are used by the paper industry for paper coatings. A very white, bright product is essential in this use, but raw kaolin as it is mined usually has a brightness of 75-80 compared with standard magnesium oxide taken as 100. By use of particle size segregation and bleaching, this brightness can be improved to 85-90 or even above. The primary sources of color in kaolins are hydrated iron oxides, ilmenite, substituted iron, tourmaline and micas. A common method of improving brightness is to treat the clay with a bleaching agent. Sodium dithionite (also known as sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4) is widely used for this purpose. Of the primary color sources, only the hydrated iron oxides are affected to any extent by chemical bleaching procedures.
According to the prior process, kaolin is slurried in water to provide about 20-25% solids. The pH is adjusted to about 3-4 or 4.5 with, for example, sulfuric acid and the sodium dithionite (also known as sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4) is added to the slurry at about 2-15 lb/ton of clay. The ferric iron, which is in solid form, is thereby reduced to the ferrous state and becomes solubilized in the process so that it can be separated from the clay by washing.
This process has given good results but it suffers from two disadvantages. The sodium dithionite is unstable in acid solution so a large excess is required for the bleaching step and even so, it is effective for only about an hour. As the dithionite becomes exhausted, aerobic oxygen again slowly re-oxidizes the remaining iron, so that at least some of the effectiveness of the dithionite is lost. The reoxidation reaction is generally known in the industry as "reversion."
Accordingly there is a need for an improved process for preventing reversion.